In operation and maintenance of a system, if a packet delay occurs, the cause thereof needs to be correctly understood to solve such a problem. In order to solve the problem, it is necessary to specifically know whether the packet delay is attributable to a delay in transmitting the packet by the sender of the packet or loss of the packet in a network followed by a retransmission of the packet by the sender and consequently a delayed arrival of the packet.
Regarding a protocol such as TCP (Transmission Control Protocol) that has a retransmission procedure among protocols in layers higher than IP (Internet Protocol), packet loss is conventionally identified using sequence numbers that monotonically increase as a clue, judging that no packet loss has occurred if the sequence numbers are consecutive numbers.
Therefore, when the sequence numbers are consecutive numbers, the packet is not recognized as having been lost, and in any case where a packet arrival is delayed from a previously received packet, it is judged that the transmitting host simply took time to transmit the packet.
Japanese Patent Application No. 2005-210515 describes a technique of identifying a retransmission or normal transmission from time-series information on packet arrivals based on sequence numbers.
However, protocols such as TCP that have a retransmission procedure, in layers higher than an IP layer, generally retransmit, after a packet loss occurs, the packet to recover from the loss. Even if a packet is lost between a transmitting host and a measuring device and the transmitting host retransmits the packet, since the measuring device can observe that the sequence numbers are consecutive numbers, there is a problem that the measuring device fails to identify the packet loss as a loss but judges it as a simple delay.
As such, the conventional method judges a loss or delay from the continuity of sequence numbers on TCP headers, and cannot thereby correctly judge the cause of delay. An object of the disclosed technique is to provide a device capable of correctly identifying whether a packet delay is attributable to a simple delay or a delay caused by a retransmission.